A tuxedo was a piece of formal clothing.
Julian Bashir bought a tuxedo during his last year at the Starfleet Medical Academy for a trip with friends to Risa. (DS9 novel: The Big Game)
Bashir later wore a tuxedo when using holosuite programs like Julian Bashir, Secret Agent and Vic Fontaine's lounge program. (DS9 episodes: "Our Man Bashir", "His Way")